Illumination system comprising a radiation source and a fluorescent material

ABSTRACT

The invention relates to an illumination system comprising a radiation source and a fluorescent material comprising at least one phosphor capable of absorbing a portion of the light emitted by the radiation source and emitting light of a wavelength different from that of the absorbed light; wherein said at least one phosphor is a green-emitting cerium-activated lutetium-aluminum-garnet of general formula (Lu1-x-y-a-bYxGdy)3(Al1-zGaz)5O12:CeaPrb wherein 0&lt;x&lt;1, 0&lt;y&lt;1, 0&lt;z£0.1, 0&lt;a£0.2 and 0&lt;b£0.1. The invention also relates to with a green-emitting cerium-activated lutetium-aluminum-garnet of general formula (Lu1-x-y-a-bYxGdy)3(Al1-zGaz)5O12:CeaPrb wherein 0&lt;x&lt;1, 0&lt;y&lt;1, 0&lt;z£0.1, 0&lt;a£0.2 and 0&lt;b£0.1.

The present invention generally relates to an illumination systemcomprising a radiation source and a fluorescent material comprising aphosphor. The invention also relates to a phosphor for use in such anillumination system.

More particularly, the invention relates to an illumination system andfluorescent material comprising a phosphor for the generation ofspecific, colored light, including white light, by luminescent downconversion and additive color mixing based on a ultraviolet or blueradiation emitting radiation source. A light-emitting diode as aradiation source is especially contemplated.

Recently, various attempts have been made to male white light-emittingillumination systems by using light-emitting diodes as radiationsources. When generating white light with an arrangement of red, greenand blue light-emitting diodes, there has been such a problem that whitelight of the desired tone cannot be generated due to variations in thetone, luminance and other factors of the light-emitting diodes.

In order to solve these problems, there have been previously developedvarious illumination systems, which convert the color of light, which isemitted by light-emitting diodes, by means of a fluorescent materialcomprising a phosphor to provide a visible white light illumination.

Previous illumination systems have been based in particular either onthe trichromatic (RGB) approach, i.e. on the mixing of three colors,namely red, green and blue, in which case the latter component may beprovided by a phosphor or by the primary emission of the LED or in asecond, simplified solution, on the dichromatic (BY) approach, mixingyellow and blue colors, in which case the yellow component may beprovided by a yellow phosphor and the blue component may be provided bythe primary emission of a blue LED.

In particular, the dichromatic approach, as disclosed in e.g. U.S. Pat.No. 5,998,925, uses a blue light-emitting diode of InGaN semiconductorcombined with an Y₃Al₅O₁₂:Ce (YAG-Ce³⁺) phosphor. The YAG-Ce³⁺ phosphoris coated on the InGaN LED, and a portion of the blue light emitted fromthe LED is converted into yellow light by the phosphor. Another portionof the blue light from the LED is transmitted through the phosphor.Thus, this system emits both blue light-emitted from the LED, and yellowlight emitted from the phosphor. The mixture of blue and yellow emissionbands is perceived as white light by an observer with a CRI in themiddle 80s and a color temperature, Tc, that ranges from about 6000 K toabout 8000 K.

However, white light LEDs based on the dichromatic approach can only beused to a limited extent for general-purpose illumination, on account oftheir poor color rendering caused by the absence of red colorcomponents.

Desirable white light lamp characteristics for general purposes are highbrightness and high color rendering at economical cost. Improvedefficiency and much improved color rendering ability is possible withthe trichromatic lamp spectrum according to the RGB-approach havingthree emission bands: red at 590 to 630, green at 520 to 560 and blue at450 nm. These wavelengths are near peaks in the CIE tristimulusfunctions, which are used to define colors.

Unfortunately, until today no green emitter with sufficient efficiencyand stability is known.

Therefore, there is a need to provide new phosphors that are excitablein the near UV-to-blue range and emit in the visible green range. It isalso desirable to provide novel phosphor blends that emit light in abroad wavelength range from green to red so that they may be combined

with UV/blue LEDs to produce white light of high efficiency and/or highcolor rendering index (“CRI”).

Thus the present invention provides an illumination system, comprising aradiation source and a fluorescent material comprising at least onephosphor capable of absorbing a portion of the light emitted by theradiation source and emitting light of a wavelength different from thatof the absorbed light; wherein said at least one phosphor is a garnet ofgeneral formula(Lu_(1-x-y-a-b)Y_(x)Gd_(y))₃(Al_(1-z)Ga_(z))₅O₁₂:Ce_(a)Pr_(b), wherein0<x<1, 0<y<1, 0<z≦0.1, 0<a≦0.2 and 0<b≦0.1.

Preferably the radiation source is selected from radiation sourceshaving an emission with a peak emission wavelength in the range of 400to 480 nm.

Preferably the radiation source is a light-emitting diode.

Another aspect of the present invention provides an illumination systemcomprising a fluorescent material comprising a garnet of general formula

(Lu_(1-x-y-a-b)Y_(x)Gd_(y))₃(Al_(1-z)Ga_(z))₅O₁₂:Ce_(a)Pr_(b), wherein0<x<1, 0<y<1, 0<z≦0.1, 0<a≦0.2 and 0<b≦0.1 at least one second phosphor;and wherein the values of x and y are selected such that 1-x-y-a-b>0.

In particular, the fluorescent material is a white light-emittingphosphor blend, comprising a garnet of general formula

(Lu_(1-x-y-a-b)Y_(x)Gd_(y))₃(Al_(1-z)Ga_(z))₅O₁₂:Ce_(a)Pr_(b), wherein0<x<1, 0<y<1, 0<z≦0.1, 0<a≦0.2, and 0<b≦0.1 and a red phosphor.

Such a red phosphor may be selected from the group of Eu(II)-activatedphosphors, selected from the group (Ca_(1-x)Sr_(x))S: Eu, wherein 0≦x≦1,and (Sr_(1-x-y)Ba_(x)Ca_(y))_(2-z)Si_(5-a)Al_(a)N_(8-a)O_(a):Eu_(z)wherein 0≦a<5, 0<x≦1, 0≦y≦1, and 0<z≦1.

The concept for white-emitting LEDs according to the invention is basedon a RGB mixture, i.e. the combination of a blue, red and green color.The essential factor is that the yellow-to-green and the red phosphorsare so broad-banded that they also have a sufficient proportion of theemission throughout the whole spectral region.

The emission spectrum of such a fluorescent material has the appropriatewavelengths to obtain together with the blue light of the LED and thegreen light of the garnet according to the invention a high qualitywhite light with good color rendering at the required color temperature.

Said white light illumination device has color coordinates substantiallyon a black body locus of a CIE chromaticity diagram.

Yielding white light emission with high color rendering is possible bythe use of red and green broad-band emitter phosphors covering the wholespectral range together with a blue-emitting LED. A blend usingwide-band phosphors according to the invention can have a relativelyhigh color rendering index, even as high as 91-93.

Another aspect of the present invention provides a phosphor capable ofabsorbing a portion of the light emitted by the radiation source andemitting light of a wavelength different from that of the absorbedlight; wherein said phosphor is a garnet of general formula

(Lu_(1-x-y-a-b)Y_(x)Gd_(y))₃(Al_(1-z)Ga_(z))₅O₁₂:Ce_(a)Pr_(b), wherein0<x<1, 0<y<1, 0<z≦0.1, 0<a≦0.2 and 0<b≦0.1.

These cerium-activated lutetium-aluminum-garnet phosphors comprise ahost garnet activated for photoluminescence or X-ray with appropriateions, which include lutetium and cerium. These garnets may also includepraseodymium and other cations including mixtures of cations asactivators.

The host garnets for these materials may be three-element (two cation)garnets such as lutetium aluminum garnet (Lu₃Al₅O₁₂), for example, ormay comprise more than three elements such as lutetium-yttrium-aluminumgarnet (Lu₃Y₃Al₅O₁₂), or lutetium-gallium-aluminum garnet((Lu₃(Al,Ga)₅O₁₂), for example.

The lutetium concentration influences the color locus of the emissionlight when used in a light source, in particular an LED. The color locusof this phosphor can be additionally fine-tuned using the ratio of thetwo concentrations Lu:Ce, which simplifies or optimizes adaptation toany further (yellow or red) phosphors in the LED.

In particular, those garnet compositions, that have praseodymium as anactivator cation present at low concentrations are particularlydesirable since such compositions show a sharp line emission also in thered region of the visible spectrum.

These phosphors are broad-band emitters wherein the visible emission isso broad that there is no 80 nm wavelength range where the visibleemission is predominantly located.

These garnets phosphors emit a broad band in the yellow-green spectralrange of the visible spectrum with very high intensity under both UV andblue excitations and thus can provide the green component in LEDs, thatemit specific colors or white light. Total conversion efficiency may beup to 90%. Additional important characteristics of the phosphorsinclude 1) resistance to thermal quenching of luminescence at typicaldevice operating temperatures (e.g. 80° C.); 2) lack of interferingreactivity with the encapsulating resins used in the device; 3) suitableabsorptive profiles to minimize dead absorption within the visiblespectrum; 4) a temporally stable luminous output over the operatinglifetime of the device and; 5) compositionally controlled tuning of thephosphor's excitation and emission properties. These garnet phosphorscan be easily synthesized.

Preferably, the fluorescent material of the illumination systemcomprises a phosphor of general formula

(Lu_(1-x-y-a-b)Y_(x)Gd_(y))₃(Al_(1-z)Ga_(z))₅O₁₂:Ce_(a)Pr_(b), wherein0<x<1, y=0, z=0, a=0.01 and b=0.

In particular, the invention relates to a specific phosphor composition(Lu_(0.99)Ce_(0.01))₃Al₅O₁₂ which exhibits a high quantum efficiency of80-90%, high absorbance in the range from 370 nm to 470 nm of 60-80% andlow losses, below 10%, of the luminescent lumen output from roomtemperature to 100° C. due to thermal quenching.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view of a tri-color white LED lamp comprising atwo-phosphor blend of the present invention positioned in a pathway oflight emitted by an LED structure.

FIG. 2 shows the coordinates of the phosphor mixture of(Lu_(0.495)Y_(0.495)Ce_(0.01))₃Al₅O₁₂ and Sr Ga₂S₄:Eu in thechromaticity diagram of the Commission Internationals de I'Eclairage(“CIE”). Blends of these phosphors may be produced to have coordinatesclose to the black body locus.

FIG. 3 discloses emission spectra of green LEDs upon excitation by ablue LED at 460 nm.

FIG. 4 discloses emission spectra of white LEDs upon excitation by ablue LED at 460 nm.

FIG. 5. discloses the excitation and emission spectra of(Lu_(0.99)Ce_(0.01))₃Al₅O₁₂.

FIG. 6. discloses the excitation and emission spectra of(Lu_(0.989)Ce_(0.01)Pr_(0.001))₃Al₅O₁₂.

FIG. 7. discloses the excitation and emission spectra of(Lu_(0.495)Y_(0.495)Ce_(0.01))₃Al₅O₁₂.

DETAILED DESCRIPTION OF THE INVENTION

The present invention focuses on a cerium-activated lutetium-aluminumgarnet as a phosphor in any configuration of an illumination systemcontaining a radiation source, including but not limited to dischargelamps, fluorescent lamps, LEDs, LDs and X-ray tubes. As used herein, theterm “radiation” encompasses radiation in the UV, IR and visible regionsof the electromagnetic spectrum.

While the use of the present phosphor is contemplated for a wide arrayof illumination devices, the present invention is described withparticular reference to and finds particular application inlight-emitting diodes, especially UV- and blue-light-emitting diodes.

The fluorescent material according to the invention comprises as aphosphor a cerium-activated lutetium-aluminum-garnet

The phosphor conforms to the general formula(Lu_(1-x-y-a-b)Y_(x)Gd_(y))₃(Al_(1-z)Ga_(z))₅O₁₂:Ce_(a)Pr_(b) wherein0<x<1, 0<y<1, 0<z≦0.1, 0<a≦0.2 and 0<b≦0.1.

This class of phosphor material is based on activated luminescence ofcubic garnet crystals. Garnets are a class of materials with the crystalchemical formula A₃B₅O₁₂.

A garnet crystal lattice has three different atomic occupying sites ofdodecahedron octacoordination, octahedron hexacoordination andtetrahedron tetracoordination, in which the A cations areeight-coordinated with oxygens and the B cations are either octahedrally(six) or tetrahedrally (four) coordinated with oxygens. The crystalstructure is cubic with 160 ions per unit cell containing eight formulaunits. In accordance with the present invention, the A cations arelutetium ions alone or in combination with yttrium and gadolinium, incombinations and with activator substitutions of cerium and possiblypraseodymium. The B cations may be aluminum and possibly gallium orother ions, again, alone, in combinations and/or with substitutions. Inparticular, it was found that with activator ions substituted in theeight-coordinated or six-coordinated sites, these garnets areluminescent in response to X-ray stimulation. A particularly importantactivator ion, which is X-ray luminescent in this host material, is theCe³⁺ ion located in eight-coordinated sites.

Replacing some of the lutetium in a cerium-activated lutetium-aluminumgarnet, Lu₃Al₅O₁₂:Ce phosphor with a smaller ion such as gadolinium Gd³⁺or yttrium Y³⁺ causes a shift in the phosphor's emission band from greento the yellow range.

Replacing some of the aluminum in a cerium-activated lutetium-aluminumgarnet, Lu₃Al₅O₁₂:Ce phosphor with a larger ion such as gallium Ga³⁺causes a shift in the phosphor's emission band from green to the bluerange.

Replacing some of the cerium in a cerium-activated lutetium-aluminumgarnet by praseodymium as a co-activator has the effect that thepraseodymium produces secondary emission that is concentrated in the redregion of the visible spectrum, instead of a typical broad-bandsecondary emission from cerium-activated lutetium-aluminum phosphorwhich is generally centered in the yellow region of the visiblespectrum. The amount of praseodymium as a co-activator can vary,depending on the amount of red color that may be required in the whiteoutput light for a particular application.

Preferably these garnet phosphors may be coated with a thin, uniformlayer of one or more compounds selected from the group formed by thefluorides and orthophosphates of the elements aluminum, scandium,yttrium, lanthanum gadolinium and lutetium, the oxides of aluminum,yttrium and lanthanum and the nitride of aluminum.

The layer thickness customarily ranges from 0.001 to 0.2 μm and, thus,is so thin that it can be penetrated by the radiation of the radiationsource without substantial loss of energy. The coatings of thesematerials on the phosphor particles may be applied, for example, bydeposition from the gas phase or a wet-coating process.

These phosphors are responsive to portions of the electromagneticspectrum of higher energy than the visible portion of the spectrum.

In particular, the phosphors according to the invention are responsiveto ultraviolet light as in fluorescent lamps and light-emitting diodes,visible light as in blue-emitting diodes, electrons (as in cathode raytubes) and X-rays (as in radiography).

The invention also relates to an illumination system comprising aradiation source and a fluorescent material comprising at least onephosphor of general formula(Lu_(1-x-y-a-b)Y_(x)Gd_(y))₃(Al_(1-z)Ga_(z))₅O₁₂:Ce_(a)Pr_(b) wherein0<x<1, 0<y<1, 0<z≦0.1, 0<a≦0.2 and 0<b≦0.1.

Radiation sources include semiconductor optical radiation emitters andother devices that emit optical radiation in response to electricalexcitation. Semiconductor optical radiation emitters includelight-emitting diode LED chips, light-emitting polymers (LEPs), organiclight-emitting devices (OLEDs), polymer light-emitting devices (PLEDs),etc.

Moreover, light-emitting components such as those found in dischargelamps and fluorescent lamps, such as mercury low and high pressuredischarge lamps, sulfur discharge lamps, and discharge lamps based anmolecular radiators are also contemplated for use as radiation sourceswith the present inventive phosphor compositions.

In a preferred embodiment of the invention, the radiation source is alight-emitting diode.

Any configuration of an illumination system which includes a LED and acerium-activated lutetium-aluminum garnet phosphor composition iscontemplated in the present invention, preferably with the addition ofother well-known phosphors, which may be combined to achieve a specificcolor or white light when irradiated by a LED emitting primary UV orblue light as specified above.

In a preferred embodiment of the invention the primary radiation sourceused is the radiation from a UV-emitting or blue-emitting LED chip.Particularly good results are achieved with a blue LED whose emissionmaximum lies at 400 to 480 nm.

An optimum has been found to lie at 445 to 460 nm, talking particularaccount of the excitation spectrum of the garnet phosphors.

A variant with particularly good color rendering is the joint use of twophosphors, namely a green emitting cerium-activatedlutetium-aluminum-garnet phosphor of general formula(Lu_(1-x-y-a-b)Y_(x)Gd_(y))₃(Al_(1-z)Ga_(z))₅O₁₂:Ce_(a)Pr_(b) wherein0<x<1, 0<y<1, 0<z≦0.1, 0<a≦0.2 and 0<b≦0.1 together with a red phosphorfrom the red-emitting europium-activated phosphors selected from thegroup of (Ca_(1-x)Sr_(x))S: Eu, wherein 0≦x≦1 and(Sr_(1-x-y)Ba_(x)Ca_(y))_(2-z)Si_(5-a)Al_(a)N_(8-a)O_(a):Eu_(z) wherein0≦a<5, 0<x≦1, 0≦y≦1 and 0<z≦1.

Preferably a europium-activated calcium strontium sulfide is used, whichis a high-chromaticity red phosphor excitable from the near UV (400 nm)to the blue-green (500 nm) with high quantum efficiency. For anoptimized use of this phosphor for luminescent conversion of primary LEDlight, it is necessary to modify the photophysical characteristics toachieve, for example, efficacy, color specifications and life time ofrelated light-emitting devices. The chromaticity and quantum efficiencyof the europium-activated strontium sulfide can be modified through thesubstitution of divalent metal ions for strontium from the listincluding Ba, Ca, Mg, and Zn.

TABLE 1 Phosphor composition λ_(max) [nm] Color point x, y SrS:Eu 6100.627, 0.372 (Sr_(1−x−y)Ca_(x)Ba_(y))₂Si₅N₈:Eu 615 0.615, 0.384(Sr_(1−x−y)Ca_(x)Ba_(y))₂Si_(5−x)Al_(x)N_(8−x)O_(x):EU 615-650*depending on x, y CaS:Eu 655 0.700, 0.303 (Sr_(1−x)Ca_(x))S:Eu 610-655*depending on x

The phosphor blend comprises a mixture of predetermined amounts andrelative proportions of garnet-structured lutetium-aluminum oxideactivated by cerium and calcium-strontium sulfide activated by divalenteuropium. The apatite structured material has a broad-band emission ofvisible radiation and the yttrium oxide material has a narrow emissionin the red-orange region of the visible spectrum, while the relativephosphor proportions are such that the composite emission of the firstphosphor layer falls approximately within the warm-white ellipse asinscribed on the x-y chromaticity diagram of the ICI system.

Especially preferred is a white emitting radiation source comprising anInGaN chip, emitting in the blue range of the visible spectrum with apeak emission at 455 nm together with a phosphor blend comprisingLu₃Al₅O₁₂:Ce and CaS:Eu with the corresponding spectral weight ratioblue:green:red=1.1:2.4:2.18 which emits white light with colorcoordinates x=0.336 and y=0.339 and a color rendering index of 83 and aluminous efficiency of about 21 lumen/Watt. The spectra of suchwhite-emitting LEDs comprising three different blends of Lu₃Al₅O₁₂:Ceand CaS:Eu are given in FIG. 4.

A detailed construction of such a light-emitting device is shown in FIG.1.

FIG. 1 is a schematic view of the device of the present invention. Thedevice comprises LED 1. LED 1 is positioned in a reflector cup 2. LED 1emits light in a pattern. A phosphor composition 4,5 is positioned inthe pattern. The phosphor composition is embedded in a resin 3. In thisexample, reflector cup 2 can modify the light pattern if light isreflected into a space not previously covered by the initial lightpattern (e.g. in the case of a parabolic reflector). It is understoodthat those skilled in the art can provide a reflector cup 2 in any shapethat optimizes reflection of light back to phosphor composition 4,5, oroptimizes the positioning of LED 1 to provide a light pattern forefficient conversion. For example, the walls of reflector cup 2 may beparabolic.

In one embodiment, the device further comprises a polymer forencapsulating the phosphor or phosphor blend. In this embodiment, thephosphor or phosphor blend should exhibit high stability properties inthe encapsulant. Preferably, the polymer is optically clear to preventsignificant light scattering. In one embodiment, the polymer is selectedfrom the group consisting of epoxy and silicone resins. A variety ofpolymers are known in the LED industry for making LED lamps. Addition ofthe phosphor mixture to a liquid that is a polymer precursor can performas encapsulation. For example, the phosphor mixture may be a powder.Introducing phosphor particles into a polymer precursor liquid resultsin formation of a slurry (i.e. a suspension of particles). Uponpolymerization, the phosphor mixture is fixed rigidly in place by theencapsulation. In one embodiment, both the composition and the LED areencapsulated in the polymer.

The phosphors are applied either separately or in a mixture. Thephosphors completely or partially absorb the light from the LED, whichemits UV/blue light, and emit it again in other spectral regions(primarily yellow and green) in a sufficiently broad-band (specificallywith a significant proportion of red) that an overall emission with thedesired color point is formed.

The color points corresponding to a black body at various temperaturesare given by the black body locus (BBL). Since the color emitted from ablack body is considered to be white, and white light is generallydesirable for a lamp, it is generally desirable that the color point ofthe light emitted from the luminescent material of a luminescent lampshould lie on or near the BBL. A portion of the BBL is shown in FIG. 2with three color temperature points highlighted on the BBL correspondingto white light-emitting LEDs, whose emission spectra are given in FIG.4.

Another figure of merit is the quality in rendering illuminated colorsof a white light-emitting radiation source, which is indicated as thecolor rendering index (CRI). A CRI of 100 is an indication that thelight emitted from the light source is similar to that from a black bodysource, i.e. an incandescent or halogen lamp. A CRI of 85 to 95 can beattained by applying a luminescent screen comprising Lu₃Al₅O₁₂:Ce andCaS:Eu in a blue-emitting LED.

FIG. 4 shows the color coordinates of a range of illumination systemsproviding white light that can be produced from various combinations ofblue LEDs and a cerium-activated lutetium-aluminum-garnet phosphor andCaS:Eu of the present invention.

More than one phosphor of the present invention may be incorporated inthe same device to provide a color adjustment. A further group ofembodiments is directed to a green-emitting illumination system withparticularly good color rendering comprising the combination of alight-emitting semiconductor component which emits primary light in theblue spectral range from 420 to 480 nm together wherein a phosphor blendcontaining a green emitting cerium-activated lutetium-aluminum garnetphosphor Lu₃Al₅O₁₂:Ce, with the corresponding spectral weight ratioblue:green is chosen from 1.0:2.4 to 1.0:3.5 which emits green lightwith color coordinates x=0.336 and y=0.339 and a color rendering indexof 83 and a luminous efficiency of about 450 lumen/Watt.

Due to the broad absorption band, the described phosphors exhibitsuitable absorption or low reflectance in the disclosed spectral rangefrom 370 nm to 480 nm for the primary light emitted by thelight-emitting semiconductor component.

The green-emitting phosphor may be combined, if appropriate, with afurther yellow or red-emitting phosphor for the production of specificcolored light and more preferably for production of white light with ahigh color-rendering index of >80.

As a result of the green-to-yellow broad-band emission, a green emissioncan be generated which is shown in some examples in FIG. 3

SPECIFIC EXAMPLE

For the phosphor synthesis of a phosphor of general formula

(Lu_(1-x-y-a-b)Y_(x)Gd_(y))₃(Al_(1-z)Ga_(z))₅O₁₂:Ce_(a)Pr_(b), wherein0<x<1, 0<y<1, 0<z≦0.1, 0<a≦0.2 and 0<b≦0.1 one or more of the startingmaterials may be oxygen-containing compounds such as oxides, nitrates,sulfates, acetates, citrates, or chlorates, which are soluble in anitric acid solution. For example, amounts of Lu₂O₃, Al(NO₃)₃9H₂O,Ce(NO₃)₃6H₂O and AlF₃ are blended and dissolved in a nitric acidsolution. The strength of the acid solution is chosen to rapidlydissolve the oxygen-containing compounds and the choice is within theskills of those skilled in the art. The nitric acid solution isevaporated. The dried precipitate is ball milled or otherwise thoroughlyblended and then calcined in a CO-atmosphere at about 1300° C. for asufficient time to ensure a substantially complete dehydration of thestarting material. The calcination may be carried out at a constanttemperature. Alternatively, the calcination temperature may be rampedfrom ambient to and held at the final temperature for the duration ofthe calcination. After an intermittent milling step the calcinedmaterial is similarly fired at 1500-1700° C. under a reducing atmospheresuch as H₂, CO, or a mixture of one of these gases with an inert gas fora sufficient time for the decomposition of the oxygen-containingcompounds to convert all of the calcined material to the desiredphosphor composition.

The resulting powder is milled on a roller bench for several hours. Themilled powder has an average particle size of 40 to 60 μm.

Its quantum efficiency is 90% and its luminous efficiency is between 430and 470 lm/W. The color point is at x=0.33 to 0.38, y=0.57 to 0.58.

TABLE 2 Phosphor composition λ_(max) [nm] Color point x, y(Lu_(0.99)Ce_(0.01))₃Al₅O₁₂ 515 + 540 0.339, 0.579(Lu_(0.989)Ce_(0.01)Pr_(0.001))₃Al₅O₁₂ 515 + 540 + 610 0.332, 0.574(Lu_(0.495)Y_(0.495)Ce_(0.01))₃Al₅O₁₂ 525 + 550 0.377, 0.570(Lu_(0.75)Gd_(0.24)Ce_(0.01))₃Al₅O₁₂ 520 + 545 0.350, 0.573

In FIGS. 5, 6, 7 of the drawings accompanying this specification, theemission spectra of various compounds are given. When excited withradiation of 355 nm wavelength, these garnet phosphors are found to givea broad-band emission, which peaks at 515 nm.

FIG. 5 of the drawings accompanying this specification shows excitationand emission spectra for the composition (Lu_(0.99)Ce_(0.01))₃Al₅O₁₂.

FIG. 6 of the drawings accompanying this specification shows excitationand emission spectra for the composition(Lu_(0.495)Y_(0.495)Ce_(0.01))₃Al₅O₁₂.

FIG. 7 shows that (Lu_(0.989)Ce_(0.01)Pr_(0.001))₃Al₅O₁₂, when scannedfor excitation is found to have a broad band (515-540 nm) excitationwith a peak extending from 515-540 nm and a side band at 610 nm.

It is also clear from the excitation spectra that these cerium-activatedlutetium-aluminum-garnet phosphors can be efficiently excited withradiation of wavelength 254 as well as 355 and 420 nm.

For manufacturing a white illumination system based on a 460 nm emittingInGaN LED, a phosphor blend comprising at least one garnet of generalformula

(Lu_(1-x-y-a-b)Y_(x)Gd_(y))₃(Al_(1-z)Ga_(z))₅O₁₂:Ce_(a)Pr_(b) wherein0<x<1, 0<y<1, 0<z≦0.1, 0<a≦0.2 and 0<b≦0.1 and one of the red phosphorsaccording to Tab. 2 is suspended into a silicone precursor. A droplet ofthis suspension is deposited onto the LED chip and subsequentlypolymerized. A plastic lens seals the LED.

1. Illumination system comprising a radiation source and a fluorescentmaterial comprising at least one phosphor capable of absorbing a portionof the light emitted by the radiation source and emitting light of awavelength different from that of the absorbed light; wherein said atleast one phosphor is a garnet of general formula(Lu_(1-x-y-a-b)Y_(x)Gd_(y))₃(Al_(1-z)Ga_(z))₅O₁₂:Ce_(a)Pr_(b), wherein0<x<1, 0<y<1, 0<z≦0.1, 0<a≦0.2 and 0<b≦0.1; and wherein values of x andy are selected such that 1-x-y-a-b>0.
 2. Illumination system accordingto claim 1, wherein the radiation source is selected from the radiationsources having an emission with a peak emission wavelength in a range of400 to 480 nm.
 3. Illumination system according to claim 1, wherein theradiation source is a light-emitting diode.
 4. Illumination systemaccording to claim 1, wherein the fluorescent material is a phosphorblend comprising a garnet of general formula(Lu_(1-x-y-a-b)Y_(x)Gd_(y))₃(Al_(1-z)Ga_(z))₅O₁₂:Ce_(a)Pr_(b) wherein0<x<1, 0<y<1 0<z≦0.1, 0<a≦0.2 and 0<b≦0.1 and at least one secondphosphor.
 5. Illumination system according to claim 1, wherein theradiation source is a blue-emitting diode and the fluorescent materialis a phosphor blend comprising a garnet of general formula(Lu_(1-x-y-a-b)Y_(x)Gd_(y))₃(Al_(1-z)Ga_(z))₅O₁₂:Ce_(a)Pr_(b), wherein0<x<1, 0<y<1, 0<z≦0.1, 0<a≦0.2 and 0<b≦0.1 and a red phosphor. 6.Illumination system according to claim 5, wherein the fluorescentmaterial is a phosphor blend comprising a garnet of general formula(Lu_(1-x-y-a-b)Y_(x)Gd_(y))₃(Al_(1-z)Ga_(z))₅O₁₂:Ce_(a)Pr_(b) wherein0<x<1, 0<y<1, 0<z≦0.1, 0<a≦0.2 and 0<b≦0.1 and a red phosphor selectedfrom the group of Eu(II)-activated phosphors.
 7. Illumination systemaccording to claim 5, wherein the fluorescent material is a phosphorblend comprising a garnet of general formula(Lu_(1-x-y-a-b)Y_(x)Gd_(y))₃(Al_(z)Ga_(z))₅O₁₂:Ce_(a)Pr_(b) wherein0<x<1, 0<y<1, 0<z≦0.1, 0<a≦0.2 and 0<b≦0.1 and a red phosphor selectedfrom the group (Ca_(1-x)Sr_(x))S:Eu, wherein 0.1≦x≦1 and(Sr_(1-x-y)Ba_(x)Ca_(y))_(2-z)Si_(5-a)Al_(a)N_(8-a)O_(a):Eu_(z) wherein0.1≦a<5, 0<x≦1, 0≦y≦1 and 0<z≦1.
 8. Phosphor capable of absorbing aportion of the light emitted by a radiation source and emitting light ofa wavelength different from that of the absorbed light; wherein saidphosphor is a garnet of general formula(Lu_(1-x-y-a-b)Y_(x)Gd_(y))₃(Al_(1-z)Ga_(z))₅O₁₂:Ce_(a)Pr_(b) wherein0<x<1, 0<y<1, 0<z≦1, 0<a≦0.2 and 0<b≦1; and wherein values of x and yare selected such that 1-x-y-a-b<0.
 9. Phosphor according to claim 8,wherein the phosphor has a coating selected from the group of fluoridesand orthophosphates of the elements aluminum, scandium, yttrium,lanthanum, gadolinium and lutetium, the oxides of aluminum, yttrium andlanthanum and the nitride of aluminum.
 10. The illumination system ofclaim 1, wherein said at least one phosphor comprises one of(Lu_(0.99)Ce_(0.01))₃Al₅O₁₂, (Lu_(0.989)Ce_(0.01)Pr_(0.001))₃Al₅O₁,(Lu_(0.495)Y_(0.495)Ce_(0.01))₃Al₅O₁₂, and(Lu_(0.75)Gd_(0.24)Ce_(0.01))₃Al₅O₁₂.
 11. The phosphor of claim 8,wherein said phosphor comprises one of (Lu_(0.99)Ce_(0.01))₃Al₅O₁₂,(Lu_(0.989)Ce_(0.01)Pr_(0.001))₃Al₅O₁,(Lu_(0.495)Y_(0.495)Ce_(0.01))₃Al₅O₁₂, and(Lu_(0.75)Gd_(0.24)Ce_(0.01))₃Al₅O₁₂.